1865 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 39, NO. 6, DECEMBER 1992

Survivability of High-T, Microwave Devices in Space Radiation Environments

E.M. Jackson,' B.D. Weaver,' W.G. Maisch,' and G.P. Summerskb

(b) University of Maryland, Baltimore County, Baltimore, MD 21228 (a) Naval Research Laboratory, Code 4615, Washington, DC 20375-5000

Abstract. A low temperature radiation damage study of first- generation passive microwave devices fabricated from high temperature cuprate superconductors has been performed in order to examine the effects of nonionizing radiation damage. Changes in performance due to radiation-induced shifts in the transition temperature and to film erosion are discussed. The results show that if these relatively simple passive devices are reasonably well shielded, they should survive in typical five- year space applications

1. INTRODUCTION

The technological potential of high transition temperature &gh-TJ superconductors is beginning to be realized in the form of passive microwave-frequency devices, such as filters, resonators, and directional couplers. High-T, superconducting devices have advantages in size, weight, and performance over nonsuperconducting devices, and are nearly ideal for appli- cation in satellite environments, where only modest active cooling is required to reach the operating temperature of about 70 K. However, radiation in the earth's geomagnetic belts can detrimentally affect devices that are not resistant to radiation. In order to investigate the suitability of high-T, devices for satellite applications, a High Temperature Superconductivity Space Experiment (HTSSE I)' is due for launch in 1993. The purpose of this article is to examine the effect of nonionizing radiation on the performance of high-T, devices intended for deployment on HTSSE I.

The earth's geomagnetic belts contain radiation that can affect the performance of electronic devices by causing both ionizing and nonionizing radiation damage. The effect of ionizing radiation on devices can be simulated by exposure to C0-60 gamma rays. This method has been shown to be generally satisfactory in reproducing the radiative effects of protons and electrons, which cause the majority of ionizing radiation damage." In doses comparable to very harsh satellite environments, ionizing radiation has been shown to have little effect on the performance of currently-available high-T, devices, provided that the superconducting material does not exhibit a high degree of granularity.' The effect of nonionizing radiation on device performance has been examined previously: but not at damage levels commensurate with typical satellite applications. It is known that nonionizing radiation, which in space occurs mostly due to incident protons and electrons, creates atomic displacements within the material. Displacements can alter many of the superconducting parameters, such as the transition tempera- ture, the critical current, the surface resistance and the microwave penetration depth.- Because the amount of

structural disorder in the superconductor lattice is critical in determining the parameters of the superconducting state, it is not surprising that high-T, superconductors can be sensitive to nonionizing (i.e., displacive) radiation.

In order to examine the effect of nonionizing radiation on the performance of shielded devices, five different micro- wave devices were tested before and after irradiation with 63 MeV protons at fluences (D of 10" and lOI3 protons/cm2 ("low fluences"). Although changes were observed in the perfor- mance of two types of devices, all devices remained opera- tional after irradiation. In order to examine the effect of nonionizing radiation on the performance of unshielded devices, several devices were exposed to fluences equivalent in nonionizing radiation damage to more than 10'' 63 MeV protons/cm2 ("high fluences"). Most of the changes in the per- formances of these devices can be attributed to shifts in the transition temperature, and to erosion of the films. The results of our findings for "low" and "high" particle fluences are discussed below.

n. LOW PARTICLE FLUENCES

For the first low-fluence set of experiments, five devices were chosen as a representative selection of the initial bank of devices proposed for HTSSE I. All are passive microwave devices made from either T1-based or YBa,Cu,O, (YBCO) superconducting films that are deposited on insulating substrates (LaAlO, or MgO) and then patterned. These devices, which include a directional coupler (Sanders), a microstrip band pass filter (MIT) and a microstrip ring resona- tor (ATLT), are described in Table I.

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JF'L coplanar low PSS filter YBCO 8.5-10.5 Sanders directional coupler T1-2212 8.5-10.5

Table 1. Description of devices irradiated for HTSSE 1.

Prior to being irradiated, devices were mounted in a Heli-tran flow-through cryostat. The surrounding vacuum chamber is fitted with stainless steel coaxial cables and microwave vacuum feed-through connectors in order to provide microwave power to the devices. Pre- and post- irradiation measurements were performed with a synthesized

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Shielding Thickness (mils of Al) 2 20 40 16 (c) 13 13

As shown in Fig. 1, a fluence of lo'' 63 MeV pro- tondcm' caused the insertion loss of the Sanders directional coupler to increase by about 0.3 dBm over most of the 8.5- 10.5 GHz band. For device input power levels higher than that of Fig. 1 (i.e., > -31 a m ) , the same fluence causes the insertion loss to increase by more than 0.3 a m . The insertion loss of the MIT filter (Fig. 2) also increased upon irradiation. For frequencies within the operating band of the filter, the total increase was about 0.07 dBm at @ = 10l2 pro- tons/cm', and about 0.15 dBm at Q, = lo" protondcm', inde- pendent of input power or frequency. The P L filter, the Loral filter, and the AT&T resonator were not measurably affected by low-fluence irradiations.

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Fig. 2. Transmitted power of the MIT microstrip filter at various fluences.

It is known that a change in transition temperature leads to a change in penetration depth, which in turn leads to a shift in resonance frequency. The constant frequency responses of the devices of Figs. 1 and 2 suggests that radiation-induced changes in T, are negligible. In fact, the change in T, at IO" 63 MeV H'/cm2 is expected to be only about 0.002 K for both YBCO and T1-based superconductors.6 Thus T, shifts cannot account for the degradation in transmitted power of the MIT

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and Sanders devices. The exact reason@) for the degradation are unknown, but could possibly be related to small changes in the loss tangent of the substrate or to a change in the surface resistance. Overall, the changes observed in device performance upon irradiation with IO" protondcm' are small. This means that throughout the duration of the experiment, the shielded high-T, devices of the HTSSE I program are likely to be resistant to damage from space radiation.

m. HIGH PARTICLE FLUENCES

Some superconducting devices, such as superconducting antennas and infrared detectors, must be exposed directly to the environment, and thus cannot be shielded against non- ionizing radiation effects. Table I1 suggests that for unshield- ed devices, the level of nonionizing radiation damage that accumulates is at least one thousand times larger than the amounts discussed in the previous section. Thus it is impor- tant to examine the effects of higher fluences on device performance. Two such effects are discussed below.

The devices used for extended radiation studies were the ATLT ring resonator, an NRL ring resonator (YBCO), an NRL five-pole filter (YBCO), and a TI-based stripline resonator (Superconductor Technologies). The AT&T ring resonator package was filled with helium gas and hermetically sealed. Displacement damage in the AT&T resonator was produced as described in the previous section, with the exceptions that the proton energy at the superconducting part of the resonator was reduced to about 10 MeV, and that some ionization of the helium gas probably occurred. The AT&T resonator was irradiated at 77 K to a maximum fluence of 1.6~10" 10 MeV protons/cm2. Since a 10 MeV proton causes about twice the amount of displacement damage in the film as does a 63 MeV proton: the maximum fluence was equivalent in displacement damage to 3 . 2 ~ 1 0 ' ~ 63 MeV protons/cm'. The remaining devices were irradiated at 300 K with 2 MeV protons incident 7 O from the c axis of the film. The supercon- ducting components of these devices were exposed directly to the proton beam, and were irradiated incrementally to a maximum fluence of 6 ~ 1 0 ' ~ 2 MeV protonskm'. A 2 MeV proton causes about 10 times as much displacement damage in the film as does a 63 MeV proton, so the maximum fluence of 2 MeV protons was equivalent in displacement damage to 6x10'' 63 MeV protons/cm2. Regardless of proton energy between 2 and 63 MeV, essentially all incident protons traversed the films without significant energy loss and came to rest in the insulating substrates or device packages.

A. FILM EROSION. In some indirectly coupled devices such as ring resonators, where one or more components of a device are ungrounded, radiation has been observed to cause film erosion regardless of the device temperature during irradiation. Erosion tends to occur mostly at the coupling leads which link the ungrounded component@) to the rest of the device. As the

coupling leads erode, the device becomes decoupled from the source of microwave power, and its insertion loss increases. Since erosion ultimately leads to device failure, it is important to know why film erosion occurs, and how it can be avoided.

Erosion is not observed when ungrounded device compo- nents are artificially grounded during irradiation. This suggests that erosion is caused by charge build-up in the substrate or film. Since charging of the insulating substrate always occurs due to proton implantation, and does not depend upon the condition of the film, erosion must be related to charging of ungrounded components of the film. Possibly, the potential gradient that arises between grounded and ungrounded superconducting parts of the film (e.g., between the ring and the microstrip insertion lines) leads to excessive film sputtering due to discharge, or to field-related evapora- tion.

The AT&T ring resonator did not exhibit signs of erosion, while at a comparable damage level the NRL ring resonator did. The simplest explanation for this observation is that the partially ionized helium gas surrounding the AT&T ring provided an effective short circuit for charge accumulat- ing in the ring. Hermetically sealing devices like ring resonators may therefore provide a hardening approach for these kinds of devices.

B. TRANSITION TEMPERATURE. Supercurrent is known to be carried in CuO, planes that are intercalated in different materials with layers of BaO, BiO, CuO, Lao, PbO, or T10. One model that provides a link between atomic structure and T, is the charge-transfer model, in which the intercalated layers act as charge reservoirs to control the electron density in the planes, thereby determining the charge valence state of the copper atoms and hence, to some degree, the transition temperature." As atomic disorder in the reservoir increases, the electron density in the planes shifts from its optimum value and causes T, to decrease. As a result of the relationship between atomic disorder and T,, the transition temperature in cuprate superconductors can be altered by nonionizing radiation. The so-called "universal curve" provides a quantitative link between radiation-induced shifts in T, for various particles and particle energies, and the amount of nonionizing energy loss per

Many parameters of the superconducting state, such as the charge carrier density and the London penetration depth, depend on the ratio of the device operating temperature T to the fluencedependent transition temperature, Tnc(@). Since most device operating characteristics (e.g., the resonance frequency and quality factor) have strong dependences on the carrier density and penetration depth," then these character- istics will depend on TIT&@) as well. In fact, it has been shown that the quality factor of an irradiated YBCO ring reso- nator depends largely on particle fluence through the radia- tion-sensitivity of T,.* This result is reinforced in Fig. 3, in which the normalized resonance frequency of the T1-based stripline resonator is plotted versus T/T&@) for various fluences of 2 MeV protons. Again, although the transition

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T/Tc(@) Fig. 3. Reduced center frequency fo(@,T)/fo(@,T) of a T1- based stripline resonator vs. TIT,(@) for various fluences of 2 MeV protons. Fluence is given in units of 10l6 protons/cm*.

temperature decreases from 100 K to 85 K with irradiation, there is no significant change in the functional dependence of the normalized resonance frequency when T,-shifts are scaled out.

IV. SUMMARY AND CONCLUSION

The passive analog microwave devices of HTSSE I are all likely to survive a five year mission in space because they are to be flown with ample shielding. However, next- generation devices will include unshielded devices like antennas, which will be subjected to rates of displacement damage that are several orders of magnitude higher than will be experienced by the HTSSE I devices. For these devices, nonionizing radiation could have serious detrimental effects on device performance. Next-generation devices will also include Josephson-type devices, which, if they behave similarly to low-T, Josephson devices, will probably be sensitive to non- displacive types of radiation damage such as transient thermal effects and digital upset. To date, there have been no reports of examinations of non-displacive radiation effects in high-T, Josephson-type devices. Testing is now underway at NRL.

Thic work waa supported by the NRL High Temperature Super- conductivity Space Erperiment. One of the authors @MJ) acknowledges the support of an NRC postdoctoral fellowship.

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